Assessment of Jessica oil spill impacts on intertidal invertebrate communities

The impact of the Jessica oil-spill on intertidal macroinvertebrate communities was studied using a reference-impact design at three tidal levels at sites where oil deposits had been reported by a shoreline assessment team, and adjacent reference sites. Faunal relationships between the various sites did not show any clear pattern related to oil exposure at the time of sampling (4-11 months post-spill). Similarly, no significant differences associated with oiling were detected for total animal density nor species richness data, nor for the abundances of common taxa. We note that post-hoc studies such as this are limited by a failure to recognise different levels of oiling impacts at different sites (oiling-site interactions). Baseline information is critical if subtle effects of oiling are to be distinguished.     

Excess heat capacity and entropy of mixing in the high-structural state (K,Ca)-feldspar binary

Low- and high-temperature heat capacities were measured for a series of synthetic high-structural state (K,Ca)-feldspars (Or–An) using both a relaxation and a differential scanning calorimeter. The data were collected at temperatures between 5 and 800 K on polycrystalline samples that had been synthesised and characterised in a previous study. Below T = 300 K, Or₉₀An₁₀, and Or₈₀An₂₀ showed excess heat capacities of mixing with maximum values of ~3 J mol⁻¹ K⁻¹. The other members of this binary (An > 20 mol%) had lower excess heat capacity values of up to ~1 J mol⁻¹ K⁻¹. Above T = 300 K, some compositions exhibited negative excess heat capacities of mixing (with maximum values of −2 J mol⁻¹ K⁻¹). The vibrational entropy at 298.15 K for Or₉₀An₁₀ and Or₈₀An₂₀ deviated strongly from the behaviour of a mechanical mixture, with excess entropy values of ~3.5 J mol⁻¹ K⁻¹. More An-rich members had only small excess vibrational entropies at T = 298.15 K. The difference in behaviour between members with An > 20 mol% and those with An ≤ 20 mol% is probably a consequence of the structural state of the (K,Ca)-feldspars, i.e., (K,Ca)-feldspars with An ≤ 20 mol% have monoclinic symmetry, whereas those with An > 20 mol% are triclinic. At T = 800 K, the vibrational entropy values were found to scatter around the values expected for a mechanical mixture and, thus, correspond to a quasi-ideal behaviour. The solvus for the (K,Ca)-feldspar binary was calculated based on the entropy data from this study in combination with enthalpy and volume of mixing data from a previous study.     

Excess heat capacity and entropy of mixing along the chlorapatite–fluorapatite binary join

The heat capacity at constant pressure, C _p, of chlorapatite [Ca₅(PO₄)₃Cl – ClAp], and fluorapatite [Ca₅(PO₄)₃F – FAp], as well as of 12 compositions along the chlorapatite–fluorapatite join have been measured using relaxation calorimetry [heat capacity option of the physical properties measurement system (PPMS)] and differential scanning calorimetry (DSC) in the temperature range 5–764 K. The chlor-fluorapatites were synthesized at 1,375–1,220°C from Ca₃(PO₄)₂ using the CaF₂–CaCl₂ flux method. Most of the chlor-fluorapatite compositions could be measured directly as single crystals using the PPMS such that they were attached to the sample platform of the calorimeter by a crystal face. However, the crystals were too small for the crystal face to be polished. In such cases, where the sample coupling was not optimal, an empirical procedure was developed to smoothly connect the PPMS to the DSC heat capacities around ambient T. The heat capacity of the end-members above 298 K can be represented by the polynomials: C _p^ClAp = 613.21 − 2,313.90T ^−0.5 − 1.87964 × 10⁷ T ⁻² + 2.79925 × 10⁹ T ⁻³ and C _p^FAp = 681.24 − 4,621.73 × T ^−0.5 − 6.38134 × 10⁶ T ⁻² + 7.38088 × 10⁸ T ⁻³ (units, J mol⁻¹ K⁻¹). Their standard third-law entropy, derived from the low-temperature heat capacity measurements, is S° = 400.6 ± 1.6 J mol⁻¹ K⁻¹ for chlorapatite and S° = 383.2 ± 1.5 J mol⁻¹ K⁻¹ for fluorapatite. Positive excess heat capacities of mixing, ΔC _p^ex, occur in the chlorapatite–fluorapatite solid solution around 80 K (and to a lesser degree at 200 K) and are asymmetrically distributed over the join reaching a maximum of 1.3 ± 0.3 J mol⁻¹ K⁻¹ for F-rich compositions. They are significant at these conditions exceeding the 2σ-uncertainty of the data. The excess entropy of mixing, ΔS ^ex, at 298 K reaches positive values of 3–4 J mol⁻¹ K⁻¹ in the F-rich portion of the binary, is, however, not significantly different from zero across the join within its 2σ-uncertainty.     

A ternary feldspar-mixing model based on calorimetric data: development and application

A mixing model for high structural state ternary feldspars in the NaAlSi₃O₈–KAlSi₃O₈–CaAl₂Si₂O₈ system is presented based exclusively on calorimetric and volumetric measurements. Comparisons with existing mixing models, which are based on phase-equilibrium experiments, reveal distinct differences. The incorporation of K into Ca-rich plagioclase and of Ca into K-rich alkali feldspar is more strongly limited by our calorimetry-based model, whereas the stability field of Na-rich feldspars is broadened. Natural feldspar assemblages from well-studied magmatic and high-grade metamorphic rocks (i.e. a teschenite sill in Scotland, the Klokken syenogabbro in Greenland, and a granulite-facies metapelite in Sri Lanka) were used to test the mixing models. It was found that the new model largely eliminates discrepancies between observed and predicted feldspar compositions that were present in earlier attempts. The reasons for the problems associated with phase-equilibrium based mixing models are discussed.     

The outbursts of dwarf novae

We present a numerical scheme for the evolution of an accretion disc through a dwarf nova outburst. We introduce a time-varying artificial viscosity into an existing smoothed particle hydrodynamics code optimized for two- and three-dimensional simulations of accretion discs. The technique gives rise to coherent outbursts and can easily be adapted to include a complete treatment of thermodynamics. We apply a two-dimensional isothermal scheme to the system SS Cygni, and present a wide range of observationally testable results.